A system in package (SiP) is a number of integrated circuits enclosed in a single module. The SiP may perform several functions of an electronic device, and is typically used inside a mobile phone, digital music player, etc. Dies in a SiP may be stacked on a substrate and connected by conductive wires. Alternatively, with flip chip technology, solder bumps may be used to join stacked chips together. This means that a multifunctional unit may be built in a multi-chip package, so that few external components need to be added to make it work.